System and method of horizontal wave measurement

1. A method of calculating wave spectra in a fluid medium utilizing a plurality of acoustic transducers, comprising: receiving echoes produced within at least two range cells, wherein the echoes are generated by a plurality of acoustic beams from the transducers, at least one of the plurality of beams having a substantially circular cross-section and at least one of the plurality of acoustic beams being positioned at a non-orthogonal angle with respect to at least one other of the plurality of acoustic beams, and the beams having the at least two range cells located at least partly therein and wherein the signals associated with the echoes comprise data indicative of a plurality of observed frequencies; for each of the plurality of observed frequencies: calculating a cross-spectral matrix, estimating the wave directional spectrum by use of the cross-spectral matrix, and calculating a non-directional height spectrum by use of the wave directional spectra.

2. The method of claim 1 , further comprising calculating a full two-dimensional directional spectrum.

3. The method of claim 1 , wherein estimating the wave directional spectrum comprises performing at least one of a maximum likelihood method and an iterative maximum likelihood method.

4. The method of claim 1 , wherein estimating the wave directional spectrum comprises performing an iterative eigenvector method.

5. The method of claim 1 , wherein the non-orthogonal angle comprises an angle in the range of 20 degrees to 40 degrees.

6. A system for measuring the directional spectrum of waves in a fluid medium having a substantially planar surface, comprising: a sonar system having a plurality of transducers for generating respective broadband acoustic beams having a substantially circular cross-section and receiving echoes from one or more range cells located substantially within the beams, at least one of the plurality of acoustic beams being generated at a non-orthogonal angle with respect to at least one other of the plurality of acoustic beams; and a computer program, executed by a processor, configured to calculate a directional spectrum associated with the waves from the received echoes, wherein the computer program further utilizes a sensitivity vector as part of the calculation of the directional spectrum.

7. The system of claim 6 , wherein said plurality of transducers is configured to generate said plurality of acoustic beams substantially in parallel to the surface of said fluid medium.

8. The system of claim 6 , wherein the sensitivity vector comprises elements for at least two range cells at different depths within an acoustic beam.

9. The system of claim 6 , wherein said sonar system is attached to a platform and said plurality of transducers is configured to generate said plurality of acoustic beams substantially in a plane horizontal from said platform.

10. The system of claim 6 , wherein the non-orthogonal angle is approximately 20 degrees.

11. The system of claim 6 , wherein the non-orthogonal angle comprises an angle in the range of 20 degrees to 40 degrees.

12. The system of claim 6 , wherein the received echoes are related to the current velocity within the range cells.

13. The system of claim 6 , wherein the transducers are arranged in a Janus configuration.

14. The system of claim 6 , wherein the transducers are in a phased array configuration.

15. The system of claim 6 , wherein the computer program is further configured to calculate a non-directional height spectrum.

16. The system of claim 6 , wherein the computer program is further configured to: calculate a cross-spectral matrix; and calculate the two dimensional directional spectrum from the cross-spectral matrix, the directional spectrum, and the sensitivity vector.

17. The system of claim 6 , wherein the fluid medium comprises water.

18. The system of claim 6 , wherein the processor is independent from the sonar system.

19. A system for calculating the directional spectrum of a wave in a fluid medium, comprising: means for generating a plurality of acoustic beams, at least one of the plurality of acoustic beams being positioned at a non-orthogonal angle with respect to at least one other of the plurality of acoustic beams, and the beams having range cells located at least partly therein; means for receiving echoes produced within an array comprising two or more of the range cells; means for processing signals indicative of the received echoes; means for storing a sensitivity vector, the vector being formed from data based on the signals and the geometry of the array of range cells, wherein the sensitivity vector comprises elements for at least two range cells at different depths within an acoustic beam; and means for estimating the directional spectrum of the wave based on the signals and the sensitivity vector.

20. The system of claim 19 , wherein the non-orthogonal angle comprises an angle in the range of 10 degrees to 89 degrees.

21. The system of claim 19 , wherein the non-orthogonal angle comprises an angle in the range of 20 degrees to 40 degrees.

22. The system of claim 19 , further comprising: means for calculating a non-directional spectrum based at least partly on the directional spectrum.

23. The system of claim 19 , wherein the means for processing of the signals indicative of the received echoes comprises: means for decoding the raw data associated with the received echoes; means for calculating the position and orientation of each of the range cells; means for calculating a mean value of current in the fluid medium; and means for numerically inverting a linear dispersion relation, wherein the linear dispersion relation relates wave frequency, water depth, and wave number.

24. The system of claim 19 , additionally comprising means for generating the sensitivity vector, the generating means comprising: means for selecting at least one range cell from at least two of the acoustic beams; means for calculating a plurality of velocity components for each of the selected range cells; means for calculating a plurality of Fourier coefficients associated with each of the velocity components; and means for calculating the sensitivity vector from the plurality of Fourier coefficients.

25. The system of claim 19 , wherein the means for estimating of the directional spectrum comprises: means for calculating the cross-spectral matrix C; means for calculating a directional spectrum at each observed frequency; means for normalizing the directional spectrum at each observed frequency; means for calculating the wave power spectrum utilizing the normalized directional spectrum; and means for scaling the normalized directional spectrum using the wave power spectrum.

26. The system of claim 19 , wherein the sensitivity vector comprises elements corresponding to surface height and pressure within the fluid medium.

27. The system of claim 19 , wherein said means for generating a plurality of acoustic beams comprises means for generating the plurality of acoustic beams in a plane substantially parallel to the surface of the fluid medium.

28. The system of claim 19 , wherein the means for estimating the directional spectrum comprises means for performing maximum likelihood processing of the signals indicative of the received echoes.

29. The system of claim 19 , wherein the means for estimating the directional spectrum comprises means for filtering directional noise components from the directional spectrum.

30. The system of claim 29 , wherein the means for filtering directional noise components from the directional spectrum comprises means for removing portions of the spectrum below a directionally-dependent threshold.

31. The system of claim 28 , wherein the means for estimating of the directional spectrum further comprises means for performing iterative maximum likelihood method (IMLM) processing.